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<meta name="description" content="1、DegradeSlot 概述Sentinel 中的熔断实现类为 DegradeSlot。DegradeSlot 的类定义如下图所示：由此可见，熔断主要实现逻辑定义在 DegradeRuleManager 的 checkDegrade 方法中。DegradeRuleManager#checkDegrade代码@1：首先从 degradeRules 熔断规则缓存中获取资源的熔断规则。 代码@2：遍">
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          <h1 class="post-title" itemprop="name headline">源码分析 Sentinel DegradeSlot 熔断实现原理</h1>
        

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        <div id="vip-container"><h2 id="1、DegradeSlot-概述"><a href="#1、DegradeSlot-概述" class="headerlink" title="1、DegradeSlot 概述"></a>1、DegradeSlot 概述</h2><p>Sentinel 中的熔断实现类为 DegradeSlot。DegradeSlot 的类定义如下图所示：<br><img src="https://img-blog.csdnimg.cn/20200412153916327.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"><br>由此可见，熔断主要实现逻辑定义在 DegradeRuleManager 的 checkDegrade 方法中。<br>DegradeRuleManager#checkDegrade<br><img src="https://img-blog.csdnimg.cn/20200412153949610.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述">代码@1：首先从 degradeRules 熔断规则缓存中获取资源的熔断规则。</p>
<p>代码@2：遍历熔断规则列表。</p>
<p>代码@3：调用熔断规则 DegradeRule 的  passCheck，如果该方法返回 false，则表示需要熔断，则抛出 DegradeException 异常。</p>
<p>即实现熔断的核心逻辑在 DegradeRule 中。</p>
<span id="more"></span>

<h2 id="2、DegradeRule-详解"><a href="#2、DegradeRule-详解" class="headerlink" title="2、DegradeRule 详解"></a>2、DegradeRule 详解</h2><p>在介绍 DegradeRule 之前我们先来看看 sentinel-dashboard 关于熔断降级规则的配置：<br><img src="https://img-blog.csdnimg.cn/20200412154228974.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"><br>我们可以直观的得知，降级规则可以根据如下三个指标进行设置：RT(响应时间)、异常比例、异常数。</p>
<h4 id="2-1-DegradeRule-类图"><a href="#2-1-DegradeRule-类图" class="headerlink" title="2.1 DegradeRule 类图"></a>2.1 DegradeRule 类图</h4><p><img src="https://img-blog.csdnimg.cn/20200412154252374.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70#pic_center" alt="在这里插入图片描述"></p>
<ul>
<li>double count<br>上面配置规则中对应的配置值，例如当降级策略为RT时，表示设置的响应时间值，其他类似。</li>
<li>int timeWindow<br>降级发生后多久进行恢复，即结束降级，单位为毫秒。</li>
<li>int grade<br>降级策略，可以选值如下：<br>1）DEGRADE_GRADE_RT<br>响应时间。<br>2）DEGRADE_GRADE_EXCEPTION_RATIO<br>异常数比例。<br>3）DEGRADE_GRADE_EXCEPTION_COUNT<br>异常数量。</li>
<li>int rtSlowRequestAmount<br>触发 RT 响应熔断出现的最小连续慢响应请求数量。</li>
<li>int minRequestAmount<br>触发熔断最小的请求数量。</li>
</ul>
<h4 id="2-2-passCheck方法详解"><a href="#2-2-passCheck方法详解" class="headerlink" title="2.2 passCheck方法详解"></a>2.2 passCheck方法详解</h4><p>根据当前请求的情况触发熔断的判断逻辑由 passCheck 方法实现。在介绍这个方法之前，我们根据该方法调用上下文得知，该方法返回 false，则触发熔断。<br>DegradeRule#passCheck</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> (cut.get()) &#123;</span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">false</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>Step1：如果当前正在处于熔断降级中，将直接返回 false，请求将被限流。</p>
<p>DegradeRule#passCheck</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">ClusterNode clusterNode = ClusterBuilderSlot.getClusterNode(<span class="keyword">this</span>.getResource());</span><br><span class="line">    <span class="keyword">if</span> (clusterNode == <span class="keyword">null</span>) &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>Step2：根据资源名称获得对应的集群类节点，有关集群限流将在后续文章中详细介绍。</p>
<p>DegradeRule#passCheck</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> (grade == RuleConstant.DEGRADE_GRADE_RT) &#123;</span><br><span class="line">    <span class="keyword">double</span> rt = clusterNode.avgRt();</span><br><span class="line">    <span class="keyword">if</span> (rt &lt; <span class="keyword">this</span>.count) &#123;</span><br><span class="line">        passCount.set(<span class="number">0</span>);</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">if</span> (passCount.incrementAndGet() &lt; rtSlowRequestAmount) &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>step3：降级策略为基于响应时间的判断规则，其核心实现关键点：</p>
<ul>
<li>首先获取节点的平均响应时间。</li>
<li>如果当前平均响应时间小于阔值，则放行，并重置 passCount 为 0。</li>
<li>如果当前平均响应时间大于阔值，但连续次数小于 rtSlowRequestAmount，依然放行，只有当连续 rtSlowRequestAmount 次响应慢才会触发降级。</li>
</ul>
<p>DegradeRule#passCheck</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line">&#125; <span class="keyword">else</span> <span class="keyword">if</span> (grade == RuleConstant.DEGRADE_GRADE_EXCEPTION_RATIO) &#123;</span><br><span class="line">    <span class="keyword">double</span> exception = clusterNode.exceptionQps();</span><br><span class="line">    <span class="keyword">double</span> success = clusterNode.successQps();</span><br><span class="line">    <span class="keyword">double</span> total = clusterNode.totalQps();</span><br><span class="line">    <span class="keyword">if</span> (total &lt; minRequestAmount) &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">    &#125;</span><br><span class="line">   <span class="keyword">double</span> realSuccess = success - exception;</span><br><span class="line">    <span class="keyword">if</span> (realSuccess &lt;= <span class="number">0</span> &amp;&amp; exception &lt; minRequestAmount) &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">    &#125;</span><br><span class="line">	<span class="keyword">if</span> (exception / success &lt; count) &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>Step4：降级策略为根据异常比例，其判断规则核心如下：</p>
<ul>
<li>分别获取成功QPS，异常QPS，总TPS。</li>
<li>如果当前总 QPS 小于 minRequestAmount，则直接返回成功，表示暂不进行熔断规则判断。</li>
<li>如果成功数小于异常数并且异常数量小于 minRequestAmount，则返回true，表示暂不进熔断规则的判断。</li>
<li>如果异常比例小于阔值，同样返回 true，表示暂不进熔断规则的判断。</li>
</ul>
<p>DegradeRule#passCheck</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">else</span> <span class="keyword">if</span> (grade == RuleConstant.DEGRADE_GRADE_EXCEPTION_COUNT) &#123;</span><br><span class="line">    <span class="keyword">double</span> exception = clusterNode.totalException();</span><br><span class="line">    <span class="keyword">if</span> (exception &lt; count) &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">    &#125;        </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>Step5：降级策略为根据异常数量，这策略只是简单的判断错误数量即可。</p>
<p>DegradeRule#passCheck</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> (cut.compareAndSet(<span class="keyword">false</span>, <span class="keyword">true</span>)) &#123;</span><br><span class="line">    ResetTask resetTask = <span class="keyword">new</span> ResetTask(<span class="keyword">this</span>);</span><br><span class="line">    pool.schedule(resetTask, timeWindow, TimeUnit.SECONDS);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>Step6：如果符合触发熔断的规则，则原子更新 cut，并且开启一个调度任务，在指定时间过后进行降级恢复。</p>
<p>Sentinel 的熔断机制实现比较简单，就介绍到这了，下一篇将介绍 Sentinel 基于集群的限流策略。</p>
<p><strong>好了，我亲爱的读者朋友，以上就是本文的全部内容了，对Sentinel 的熔断实现原理是否已Get。原创不易，莫要白票，请你为本文点赞个吧，这将是我写作更多优质文章的最强动力。</strong></p>
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